The gas cap has a "ratcheting" mechanism built into it. When you turn the cap to lock it you should hear a clicking sound. I tell people to turn it another 180 degrees after they hear the first click.Many auto repair shops have a "smoke machine". It is used to put a non flammable smoke into the gas tank to check for leaks. I wonder is the neck of the gas tank damaged that the cap seals on or is the cap its self defective?